1. Field of the Invention
The present invention relates to high-voltage MOS transistors and, more particularly, to a vertical high-voltage MOS transistor and a method of forming the MOS transistor with improved on-state resistance.
2. Description of the Related Art
A metal oxide semiconductor (MOS) transistor is a well-known semiconductor device that has a source, a drain, a body that has a channel region that lies between and touches the source and drain, and a gate that lies over and is isolated from the channel region by a gate dielectric layer. There are two types of MOS transistors: an NMOS transistor that has n+ source and drain regions with a p-type channel region, and a PMOS transistor that has p+ source and drain regions with an n-type channel region.
In operation, when the source and the body are grounded, a positive voltage is placed on the drain to set up a drain-to-source electric field, and a voltage is placed on the gate that is greater than a threshold voltage, a current flows from the drain to the source. When the voltage placed on the gate is less than the threshold voltage, such as when the gate is pulled down to ground, no current flows.
Current-generation MOS transistors are commonly used in low-voltage environments that range from, for example, 1.2V to 5V. In contrast, a high-voltage MOS transistor is a transistor that operates with voltages in the range of, for example, 10V to 400V. High-voltage MOS transistors are bigger than low-voltage MOS transistors due to the need for a thicker layer of gate dielectric below the gate, and a drain drift region that lies between the drain and the channel region. In operation, the drain drift region reduces the magnitude of the drain-to-source electric field. The drain drift region, which is relatively large, has the same conductivity type as the drain, but a lower dopant concentration than the drain.
When formed to be structurally similar to a low-voltage MOS transistor, a high-voltage MOS transistor consumes a significant amount of silicon real estate due to the space required by the relatively large drain drift region. In an effort to reduce the required space and produce a more compact transistor, some high-voltage MOS transistors are vertically arranged so that the source lies above the drain.
One type of vertical high-voltage MOS transistor uses a pair of trench structures that lie on opposing sides of a drain drift region, which has a lower end region that touches the drain, and an upper end region that touches the body region. The trench structures have conductive field plates that can alter the electric field across the drain drift region when a voltage is placed on the field plates.